🏃 Locomotion & Muscle Contraction

Movement and locomotion in vertebrates are executed through the coordinated mechanical coupling of the musculoskeletal system, sliding myofibrillar sarcomere proteins, and motor neuron depolarization.

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1. 🔬 Sarcomere Ultrastructure (The Functional Unit of Contraction)

A sarcomere is the segment between two successive Z-lines:

        │◄──────────────────────── SARCOMERE ────────────────────────►│
        Z-Line                       M-Line                       Z-Line
        │     ────── Thin Actin ──────► ◄────── Thin Actin ──────     │
        │    ════════════ Thick Myosin Filament ════════════          │
        │     ────── Thin Actin ──────► ◄────── Thin Actin ──────     │
        │◄── I-Band ──►│◄────────────── A-Band ──────────────►│◄── I ─►│
                       │◄──── H-Zone ────►│

Sarcomere Feature	Composition & Structural Identity	State During Muscle Contraction
A-Band (Anisotropic)	Thick Myosin filaments + overlapping thin Actin ends	Length Remains CONSTANT
I-Band (Isotropic)	Thin Actin filaments only; bisected by Z-line	SHORTENS / Narrows
H-Zone	Central region of A-band containing thick Myosin only (no actin overlap)	REDUCES / Disappears
Distance Between Z-Lines	Sarcomere length	Decreases (Contraction)

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2. ⚡ The Sliding Filament Theory (Huxley & Hanson, 1954)

  Action Potential arrives at Neuromuscular Junction (Motor End Plate)
         │
         ▼ [Acetylcholine release -> Depolarizes Sarcolemma -> T-Tubules]
  Sarcoplasmic Reticulum releases Ca2+ into Sarcoplasm
         │
         ▼
  Ca2+ binds to Troponin-C ──► Tropomyosin shifts, exposing Myosin-binding active sites on Actin
         │
         ▼
  Myosin Head (with ADP + Pi) binds Actin Active Site ──► Forms CROSS-BRIDGE
         │
         ▼ [Power Stroke: Pi & ADP released]
  Myosin head rotates, pulling Actin filaments inward toward M-line (Sarcomere shortens)
         │
         ▼ [New ATP molecule binds to Myosin Head]
  Cross-Bridge BREAKS ──► ATP hydrolyzed by Myosin ATPase to ADP + Pi (Head re-cocks)

Rigor Mortis

Following clinical death, cellular ATP synthesis ceases. Without fresh ATP binding, myosin heads remain permanently locked to actin cross-bridges, causing progressive muscle stiffness (Rigor Mortis).

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3. 🦴 Human Skeletal System ($206\text{ Bones}$) & Joints

$$\text{Human Skeleton }\mathbf{(}\mathbf{206}\mathbf{)}\mathbf{=}\text{Axial Skeleton (80)}\mathbf{+}\text{Appendicular Skeleton (126)}$$

• Axial Skeleton (80): Skull ($22+6\text{ ear ossicles}+1\text{ hyoid}=29$), Vertebral Column ($26$: $C_7,T_{12},L_5,S_{(5)=1},C{o}_{(4)=1}$), Ribs ($12\text{ pairs}=24$), Sternum ($1$).
• Joint Classifications:
  1. Fibrous / Synarthrodial (Immovable): Sutures between cranial flat bones.
  2. Cartilaginous / Amphiarthrodial (Slightly Movable): Pubic symphysis, Intervertebral discs.
  3. Synovial / Diarthrodial (Freely Movable with Synovial Cavity & Fluid):
     • Ball and Socket: Shoulder joint (Humerus-Glenoid), Hip joint (Femur-Acetabulum).
     • Hinge Joint: Knee, Elbow, Interphalangeal joints.
     • Pivot Joint: Atlas-Axis ($C_1-C_2$, allows head rotation).
     • Gliding Joint: Between carpals of the wrist.
     • Saddle Joint: Between carpal and metacarpal of the human thumb.